184 COLLEGE BOTANY 



Precipitation and Plant Growth. It is very evident that 

 plant growth depends in a great measure on the amount of rain- 

 fall, its distribution throughout the year, and the water-retaining 

 character of the soil. Most of the water that falls on the earth 

 runs off into the streams and thence into the lakes and seas and 

 is lost so far as land plants are concerned ; a considerable amount 

 percolates through the soil and it is also lost for land plants ; 

 a considerable amount evaporates from the surface of the soil 

 and is lost. Therefore, a comparatively small amount of the 

 rainfall is really available for plant growth. 



It is well known that the amount and distribution of rainfall 

 varies greatly in different localities. A study of the precipita- 

 tion map of the United States will demonstrate this point quite 

 clearly. A study of the crop maps shows that our crop produc- 

 tion coincides very closely with these rainfall areas (Figs. 100 

 and 101). The plants that require large amounts of water are 

 restricted to the areas of greatest rainfall or to the areas where 

 irrigation is practical. Of course, temperature and soil and 

 other factors are important and will be taken up later. The lack 

 of rainfall is overcome in some of the arid sections of the 

 country by means of irrigation. 



Ecological Classification and Crop Production. The 

 amount of available water is one of the most important con- 

 trolling factors in plant growth ; in fact, life and growth are 

 absolutely impossible without water. However, the water re- 

 quirements for different species of plants are quite different. 

 Therefore, it is very natural that those species of plants which 

 have practically the same water requirements will be found 

 growing in the same locality ; in fact, in very close association ; 

 although they may be very different in character and in appear- 

 ance. On this basis, plants have been classified into three large 



